1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copier or a printer, that employs an electrostatic recording system or an electrophotographic recording system, and relates in particular to an image forming apparatus that comprises a speed controller for an intermediate transferring body or a conveying belt, and a control method therefor.
2. Related Background Art
A full color image forming apparatus is well known that comprises multiple image forming parts for forming an image on a photosensitive drum using an electrophotographic process, wherein images formed by these image forming parts are sequentially superimposed and transferred to the same transfer medium, or to an intermediate transferring body, so as to obtain a full color image. When a full color image is formed on an intermediate transferring body, this image is thereafter collectively transferred to the transfer medium (secondary transfer).
For such an image forming apparatus, when the speed changes at which a conveying belt, used to convey either a transfer medium, or an intermediate transferring body is driven, the position whereat an image is superimposed is shifted, and a so-called misregistration occurs. It should be noted that the speed of a conveying belt varies due to an eccentricity of a drive roller that drives the conveying belt or to an uneven belt thickness, and also due to thermal expansion of the drive roller.
Example fluctuations in the speed of a conveying belt are shown in FIG. 3. In this example, a periodical speed variation that is caused by the eccentricity of the drive roller or the uneven thickness of the belt is superimposed with the speed variation caused by temperature. Therefore, the average speed of the conveying belt is gradually changed due to the rise of the temperature inside the image forming apparatus as it is operated and due to the environmental change of the apparatus. That is, as is described above, even when the adjustment is made to remove detected misregistration, the misregistration in the sub-scanning direction that depends on the speed of the conveying belt is increased in proportion to the degree of the temperature change.
As one method whereby the conveying speed or the traveling speed of the intermediate transferring body is stabilized as much as possible to prevent the occurrence of misregistration, there is a technique for controlling a motor that rotates a drive roller for driving a conveying belt, so that, based on the speed of the conveying belt that is detected, a target conveying speed is set. According to this technique, in order to detect the rotational speed at which a driving roller is driven in synchronization with a conveying belt, an optical sensor is employed that generates a pulse signal synchronized with the rotation of the driving roller (outputs a pulse for each revolution).
For the full color image forming apparatus, there is a well known technique that performs image forming by setting the image forming speed and the conveying speed to a low level (1/2-speed or 1/4-speed) in order to satisfactorily fix an image to a sheet other than a regular sheet, such as a thick sheet or an OHT. However, when this conveying belt speed stabilization technique is employed in a low speed mode, the following problems have arisen.
When the speed of the conveying belt or the intermediate transferring body is to be detected in the low-speed mode (1/2-speed, 1/3-speed or 1/4-speed) within the same period as for detection for the 1/1-speed, the total fetched number of pulses that are generated for each revolution of the driven roller is smaller than the number of pulses fetched at the 1/1-speed, and the conveying belt speed would be corrected under the adverse affect of a variance of the pulse intervals. As a result, the belt speed becomes unstable. Accordingly, the belt speed differs between when the misregistration is detected and when the printing is actually performed, and misregistration occurs in the sub-scanning direction. Further, at the sequential printing, the misregistration distance in the sub-scanning direction differs for each printed sheet. In addition, when the number of pulses fetched for each revolution of the driven roller operated at a low speed is to be equaled to that at the 1/1-speed, the first printing period is extended, and the performance of the image forming apparatus is deteriorated.
To resolve these problems, it is an object of the present invention to provide a color image forming apparatus that can improve the image forming operation by reducing misregistration on an image by precisely performing speed correction for low-speed mode printing and a control method therefor.
Another object of the present invention is to provide an image forming apparatus comprising:
an image forming apparatus as an image forming means for forming an image;
a conveying member as conveying means for conveying a transfer medium to which the image is to be transferred;
a driver as driving means for driving the conveying member at one of multiple speeds, which at least include a standard speed mode and a low speed mode;
a detector as detecting means for detecting a conveying speed for the conveying member; and
a controller as control means for controlling a drive condition of the conveying member based on the detection results obtained by the detector,
wherein the controller employs the conveying speed of the conveying member in the standard speed mode to determine the drive condition of the conveying member in the low speed mode.
A further object of the present invention is to provide an image forming apparatus comprising:
an image forming apparatus as an image forming means for forming an image;
an intermediate transferring body to which the image is to be transferred;
a driver as driving means for driving the intermediate transferring body at one of multiple speeds, which at least include a standard speed mode and a low speed mode;
a detector as detecting means for detecting a traveling speed for the intermediate transferring body; and
a controller as control means for controlling a drive condition for the intermediate transferring body based on the traveling speed detected by the detector,
wherein the controller employs the traveling speed detected in the standard speed mode for the intermediate transferring body to determine the drive condition for the intermediate transferring body in the low speed mode.
A still further object of the present invention is to provide a control method, for an image forming apparatus that drives a conveying member as conveying means for conveying, in one of multiple speed modes, including at least a standard speed mode and a low speed mode, a transfer medium to which a formed image is to be transferred, comprising:
a detection step of detecting, in the standard speed mode, a conveying speed for the conveying member; and
a control step of employing the conveying speed detected in the detection step to control a drive condition for the conveying member,
wherein, in the control step, based on the conveying speed detected in the standard speed mode for the conveying member, a drive condition in the low speed mode is determined for the conveying member.
A yet further object of the present invention is to provide a control method, for an image forming apparatus that is capable of driving an intermediate transferring body, to which a formed image is to be transferred, in one of multiple speed modes, including at least a standard speed mode and a low speed mode, comprising:
a detection step of detecting a traveling speed for the intermediate transferring body; and
a control step of employing the traveling speed detected in the detection step to control a drive condition for the intermediate transferring body,
wherein, in the control step, based on the traveling speed detected in the standard speed mode for the intermediate transferring body, a drive condition in the low speed mode is determined for the intermediate transferring body.
Other objects, configurations and effects of the present invention will become obvious during the course of the following detailed description, given while referring to the accompanying drawings.